Seat back



FIG. 1 is a front, right and top perspective view of a seat back showing my new design;

FIG. 2 is a front view thereof;

FIG. 3 is a rear view thereof;

FIG. 4 is a left side elevational view thereof;

FIG. 5 is a right side elevational view thereof;

FIG. 6 is a top plan view thereof;

FIG. 7 is a bottom plan view thereof; and,

FIG. 8 is a view similar to FIG. 1 further showing environment of use of the seat back.

The broken lines in FIG. 8 are environmental only and form no part of the claimed design. 

The ornamental design for a seat back, as shown and described. 